(1) Field of the Invention
The present invention relates to a wavelength dispersion (chromatic dispersion) compensating apparatus for compensating for wavelength dispersion and wavelength dispersion slope occurring in signal lights of respective wavelengths transmitted on optical fibers, in an optical communication of a wavelength division multiplexing (WDM) system.
(2) Description of the Related Art
In a conventional optical fiber communication system for transmitting information using light, a transmitter sends out an optical pulse via an optical fiber to a receiver. However, wavelength dispersion, also known as “chromatic dispersion”, occurring in the optical fiber deteriorates the signal quality in the system.
Specifically, due to wavelength dispersion characteristics of the optical fiber, propagation speed of a signal light in the optical fiber is dependent on a wavelength of the signal light. For example, when an optical pulse having a long wavelength (for example, an optical pulse of a wavelength indicating red color) is propagated at a speed higher than an optical pulse having a short wavelength (for example, an optical pulse of a wavelength indicating blue color), the wavelength dispersion in the signal light is called normal dispersion. Conversely, when an optical pulse having a short wavelength (for example, a blue pulse) is propagated at a speed higher than an optical pulse having a long wavelength (for example, a red pulse), the wavelength dispersion in the signal light is called abnormal dispersion.
Accordingly, in the case where a signal light contains a red pulse and a blue pulse when sent out from a transmitter, the signal light is separated into the red and blue pulses while being propagated through the optical fiber, and then each separated pulse is received by a light receiver at different times.
As another example of optical pulse transmission, in the case where a signal light having wavelength components which are consecutive from blue to red is transmitted, the respective components are propagated through the optical fiber at different speeds, and thus the time-width of pulse waveform of the signal light is extended inside the optical fiber, resulting in the distortion of pulse waveform. Since all pulses include components within a limited wavelength range, this wavelength dispersion is extremely common in optical fiber communications systems.
Particularly in a high-speed optical fiber communication system, it is necessary to compensate for the wavelength dispersion in order to obtain high transmission performance.
In order to compensate for this wavelength dispersion, the optical fiber communication system needs to be provided with a “reciprocal dispersion component” which gives wavelength dispersion reciprocal to the wavelength dispersion occurring in the optical fiber to the optical pulse. In the conventional apparatuses, there exists the one capable to be used as this reciprocal dispersion component. For example, a dispersion compensating fiber has a specific cross-sectional refractive index distribution, and is capable to give wavelength dispersion reciprocal to wavelength dispersion occurring in a normal transmission path fiber to the optical pulse. Therefore, it can be used as the reciprocal dispersion component.
However, the dispersion compensating fiber is expensive in manufacturing cost, and it is necessary to make the fiber length thereof relatively long in order to sufficiently compensate for the wavelength dispersion occurring in the transmission path fiber. For example, to completely compensate for the wavelength dispersion occurred in the transmission path fiber of 100 km, a dispersion compensating fiber of between 20 km and 30 km is required. Therefore, there are caused problems of a large optical loss, and a large size.
Furthermore, in addition to the above dispersion compensating fiber, a chirped fiber grating can be used as the reciprocal dispersion component to compensate for the wavelength dispersion. A fiber grating is formed with, in the core thereof, a grating structure whose refractive index is changed at a half-wavelength period, using a phenomenon in which the refractive index of core-doped germanium oxide is changed with the ultraviolet beam irradiation. The chirped fiber grating is designed such that, by gradually changing the grating intervals in a longitudinal direction of the above fiber grating to reflect long wavelength components at long distances so that the long wavelength components are propagated for distances longer than propagation distances of short wavelength components. Accordingly, the chirped fiber grating can also give the reciprocal dispersion to the optical pulse.
However, since the chirped fiber grating has a reflective band of very narrow wavelength width, it is difficult to realize a sufficient reflective band for compensating for wavelength dispersion of a light containing a large number of wavelengths such as a WDM light. It is possible to connect in cascade multiple chirped fiber gratings to realize a reflective band corresponding to the WDM light. However, there is a problem in that a system applying such a reciprocal dispersion component is expensive.
As one of conventional techniques to resolve these problems, there has been proposed an apparatus in which, for example, a device called a virtually imaged phased array (hereafter referred to as “VIPA”) as shown in FIG. 18 is utilized to compensate for the wavelength dispersion occurring in the WDM light (refer to Japanese Unexamined Patent Publication No. 2002-258207).
This apparatus includes a VIPA plate 110 which demultiplexes the WDM light into a plurality of optical beams capable to be spatially discriminated from each other (for example, traveling directions of optical beams being different from each other), according to wavelengths, and a light return apparatus which reflects a light output from the VIPA plate 110 to return it to the VIPA plate 110 again. The VIPA plate 110 includes a transparent member 111 having parallel first and second planes 112 and 113. The first plane 112 of the transparent member 111 has a characteristic to reflect a light at the reflectance of approximately 100% except for a transmission area 114 formed on a part thereof, and the light passes through the transmission area 114, to be input to and output from the transparent member 111. The second plane 113 of the transparent member 111 has a characteristic to reflect a light at the reflectance lower than 100%. The light having passed through the transparent area 114 to be input to the transparent member 111 is multiple-reflected repeatedly between the first and second planes 112 and 113. At this time, a few percent of the light is transmitted through the second plane 113 to be emitted to the outside of the transparent member 111. The lights transmitted through the transparent member 111 interfere mutually and generate a plurality of optical beams capable to be spatially discriminated, traveling directions of which are different from each other, according to wavelengths. The VIPA plate 110 is a device with angular dispersion, since the output lights can be discriminated from each other according to traveling angles thereof. The light return apparatus reflects the output light from the VIPA plate 110, to return it to the VIPA plate 110. The light reflected by the light return apparatus is transmitted through the second plane 113 to be input to the transparent member 111, and is multiple-reflected repeatedly between the first and second planes 112 and 113, to be output to an input path from the transparent area 114.
Furthermore, the above VIPA plate 110 has the same wavelength as the wavelength of the input light, and has a function of generating a plurality of output lights having different orders of interference. The light return apparatus is provided with a structure in which the output light of one order of interference is returned to the VIPA plate 110, but the output lights of other orders of interference are not returned to the VIPA plate 110. Thus, only the light corresponding to one order of interference passes through the VIPA plate 110, to be output to the input path.
Moreover, the above light return apparatus is provided with a lens 160 and a mirror 170, as a specific configuration thereof. The lens 160 has a function of condensing the lights output from the VIPA plate 110 to the different directions according to the wavelengths, onto different positions on the surface of the mirror 170, and also orienting the lights reflected by the mirror 170 to the VIPA plate 110. The mirror 170 is located such that the light traveling from the VIPA plate 110 to the lens 160, and the light returning from the lens 160 to the VIPA plate 110 are propagated in parallel and opposite directions, and are prevented from being overlapped with each other. As a result, the lights of respective wavelengths reflected by the light return apparatus are propagated for different distances, so that the wavelength dispersion of the WDM light is compensated for.
As described in the above, the apparatus using the VIPA plate 110 has the angular dispersion function similar to a diffraction grating, and is capable to compensate for the wavelength dispersion occurring in WDM light. In particular, a VIPA-type wavelength dispersion compensating apparatus has a feature capable to generate considerable angular dispersion, and accordingly, can readily provide a practical reciprocal dispersion component.
A practical reciprocal dispersion component for use in a WDM transmission system is required to serve the following special needs.
A wavelength dispersion characteristic of an optical fiber generally in practical use is not constant depending on wavelength as shown in FIG. 19 for example, and frequently has a slightly positive inclination (wavelength dispersion is increased as the wavelength becomes longer). Such an inclination of wavelength dispersion is referred to as wavelength dispersion slope, or second order wavelength dispersion. Specifically, in a typical 1.3 μm zero-dispersion single mode fiber (SMF) as shown by the dotted line in FIG. 19, for a light of wavelength 1550 nm, the wavelength dispersion per 1 km is +16.79 ps/nm/km, while the wavelength dispersion slope per 1 km is 0.057 ps/nm2/km. In the case where the necessary wavelength bandwidth is 35 nm for example, a variation in wavelength dispersion of approximately +2 ps/nm occurs within such a wavelength band.
The solid line in FIG. 19 indicates a characteristic of E-LEAF optical fiber manufactured by Corning Inc. In this E-LEAF optical fiber, for the light of wavelength 1550 nm, the wavelength dispersion is 3.852 ps/nm/km, and the dispersion slope is 0.083 ps/nm2/km. On the other hand, the broken line in FIG. 19 indicates a characteristic of TW-RS optical fiber manufactured by Lucent Inc., and for the light of 1550 nm wavelength, the wavelength dispersion is 4.219 ps/nm/km, and the dispersion slope is 0.045 ps/nm2/km. Furthermore, the respective wavelength dispersion characteristics in FIG. 19 are practically not linear, and strictly speaking, the inclinations (wavelength dispersion slope) of the wavelength dispersion are not constant. However, such third-order wavelength dispersion can be neglected since it presents very few problems at a transmission speed of approximately 40 Gb/s.
Here, if the wavelength dispersion in the optical fiber transmission path is considered in practice, as shown in FIG. 19, the wavelength dispersion and wavelength dispersion slope per unit length, are determined depending on the type of optical fiber used as the transmission path, and next, the actual wavelength dispersion and wavelength dispersion slope are determined depending on the length of the optical fiber (transmission distance). In the case where such actual wavelength dispersion in the optical fiber transmission path is compensated for with the reciprocal dispersion component, it is desirable to set the wavelength dispersion to be variable within a certain range, as a characteristic of the reciprocal dispersion component. This is because the types and transmission distances of the optical fiber are in infinite variety depending on the transmission speed and wavelength band of the transmission system, the timing at which the optical fiber was installed, and the conditions of the installation site.
Moreover, in the case of WDM transmission, it is insufficient even if only the wavelength dispersion can be compensated as described above, and the wavelength dispersion slope also becomes problematic. This is because, even if the dispersion can be compensated with a wavelength of a given signal channel, if the wavelength dispersion of the reciprocal dispersion component is constant, the wavelength dispersion cannot be compensated completely with a wavelength of a different signal channel. It is therefore desirable that the characteristic of the reciprocal dispersion component for WDM transmission has the wavelength dispersion slope. Furthermore, as described above, since the transmission distances are in infinite variety, and the wavelength dispersion slope is varied together with the wavelength dispersion in proportion to the length of the optical fiber, it is desirable that the wavelength dispersion slope is also set to be variable within a certain range.
However, a value of the wavelength dispersion slope to be given, is not determined uniquely with a wavelength dispersion value. This is because, not only the wavelength dispersion but also the wavelength dispersion slope are varied if the type of optical fiber is changed, as is apparent from FIG. 19. That is to say, in the case of WDM transmission, in order to compensate for the wavelength dispersion in the optical fiber transmission path by the reciprocal dispersion component, it is most desirable to set the wavelength dispersion and the wavelength dispersion slope to be variable independently within certain ranges.
However, although the wavelength dispersion can be set to be variable within a required range by the conventional reciprocal dispersion component as described above, it cannot have been realized that the wavelength dispersion and the wavelength dispersion slope are varied independently, as described above.
Specifically, for the dispersion compensating fiber, since it is possible to design an index profile having the reciprocal dispersion slope, a dispersion compensating fiber having the required wavelength dispersion and wavelength dispersion slope can be realized. However, in order to vary the wavelength dispersion and the wavelength dispersion slope independently, a dispersion compensating fiber having a variety of index profiles, and a variety of lengths, is necessary. Therefore, such a dispersion compensating fiber is not practical. Moreover, as described above, such a dispersion compensating fiber has problems of high cost, large loss, large size and the like.
Furthermore, in the chirped fiber grating, as with the dispersion compensating fiber, if the chirp design of chirped fiber grating is optimized, the reciprocal dispersion slope can be given. However, in order to change the value thereof, a variety of index profiles and a variety of lengths, are necessary. Therefore, such a chirped fiber grating is not practical. Even if the temperature is changed to vary the wavelength dispersion and the wavelength dispersion slope, since the wavelength dispersion slope value is determined uniquely with the wavelength dispersion value, the wavelength dispersion and the wavelength dispersion slope cannot be varied independently. Additionally, as described above, it is also hard for the chirped fiber grating to obtain the sufficient wavelength bandwidth for compensating for the light having a large number of wavelengths such as WDM light.
Moreover, in the reciprocal dispersion components using conventional diffraction gratings, there is a possibility of varying the wavelength dispersion and the wavelength dispersion slope independently to a certain extent depending on how the diffraction gratings are combined. However, since there is a limit in the angular dispersion obtainable within practical dimensions of typical diffraction gratings other than the VIPA, it is difficult to give the sufficiently large angular dispersion capable to compensate for the wavelength dispersion of relatively large value, which occurs in the optical fiber communication system. Therefore, such a reciprocal dispersion component is not practical.